1. Field of the Invention
The invention relates to an image reading apparatus for reading an image, including a reference member and an image reading device having a plurality of light receiving elements and reading the reference member by the light receiving elements with respect to a plurality of colors, in order to obtain white level information for the respective colors based on at least which data of the read image is corrected.
2. Description of Related Art
As an image reading apparatus incorporated in a copy machine, a scanner, or a multifunctional apparatus having functions thereof, there are known a flatbed scanner, and an apparatus in which an image reading device, e.g., a CCD (Charge Coupled Device) or a CIS (Contact Image Sensor), is disposed at a predetermined relative position with respect to a mechanism called ADF (Auto Document Feeder) that operates to automatically feed document sheets, which is a document in the form of cut sheets, from a document supply tray to a catch tray along a feed path, in order to read an image on each document sheet while the document sheet is fed.
For instance, there is known an image reading apparatus that has a cover structure or a document holding member including an ADF, a reading table having at an upper surface thereof a platen glass, an image reading device disposed in the reading table, and a document supply tray on which document sheets each with an image thereon are stacked. On the platen glass, a fed-document reading area is defined, and the ADF feeds the document sheets one by one from the document supply tray to the fed-document reading area, so that the image reading device in the reading table operates to read the image on the document sheet as passing across the fed-document reading area on the platen glass. However, the image reading apparatus can also function as a flatbed scanner, that is, a stationary-document reading area is also defined on the platen glass, and the image reading apparatus can be used in such a manner that a document with an image thereon is placed on the platen glass and the document holding member is closed to hold down and fix in position the document, and then the image reading device is operated to read the image on the document.
The CCD or CIS used as an image reading device or image sensor has light sources and light receiving elements, and reads an image on a document as follows. The light sources emit light of a plurality of colors, namely, red (R), green (G), and blue (B), toward the platen glass so that the emitted light of respective colors is transmitted through the platen glass and reflected by a surface of the document. The reflected light is received by the light receiving elements that convert the received light of the respective colors RGB into electrical signals. Since undesirable variations occur with regard to operations of the light sources and the light receiving elements of such an image reading device, namely, amounts of light of RGB emitted from the light sources and spatial distributions of the RGB light, and photographic sensitivities of the light receiving elements, a control operation called “shading correction” is implemented to correct image data obtained by reading the image on the document using the image reading device. More specifically, the shading correction is an operation such that the amount of the RGB light as emitted from the light sources is adjusted with respect to a white-colored reference member, and then the reference member is read to obtain white level information and black level information for each of RGB while the light sources emit RGB light of the amounts adjusted as described above, so that the image data read thereafter is corrected based on the thus obtained white and black level information. For instance, the conventional image reading apparatus described above has the reference member on an under side of a partitioning member that is disposed to divide an upper surface of the platen glass into two areas, namely, the fed-document reading area and a stationary-document reading area, and the apparatus is set to adjust the amount of the RGB light emitted from the light sources with respect to the reference member, and obtain reference data for each of RGB, prior to reading the image.
Where a foreign particle, a flaw, or the like is on the reference member, a quantity obtained by reading the reference member as the white level information shows a local abrupt variation resulting from the foreign particle, flaw, or the like. When data of a read image is subjected to the shading correction based on such white level information, a line extending in an auxiliary scanning direction, called shading line, occurs in the read image. JP-A-6-22133 discloses a technique to take measures to this problem, that is, to detect an abnormality in any set of data of a quantity obtained as the white level information for each of RGB, which abnormality results from the foreign particle, flaw, or the like, and to replace abnormal data in the white level information, if any, with data in another set of data of the white level information not including the abnormal data, which data is positionally corresponding to the abnormal data.
However, the white level information obtained by reading the reference member is different from color to color. That is, in the image reading device, a plurality of lenses are disposed for concentrating the RGB light, and a plurality of the light receiving elements are provided for receiving the RGB light, and the chromatic aberration of the lenses and the sensitivity characteristic of the light receiving elements differs from color to color, thereby making the white level information different from color to color. Thus, simply replacing an abnormal part of the abnormal white level information with a corresponding part of the normal white level information results in inaccuracy of the shading correction. More specifically, a local abrupt variation occurs in the read image, namely, a place in the read image that corresponds to an abnormal part of the white level information is unnaturally different from the other part of the read image. Thus, the problem of the shading line seen in the read image is not solved.